Heretofore, CD type optical disks such as CD-Rs or CD-RWs have been used as optical recording media. In recent years, DVD type optical disks having a large capacity such as DVD-RAMs or DVD-Rs are more popularly used as optical recording media.
Under the circumstances, a CD/DVD compatible optical head device has been practically used for recording or reproducing an information of CD and DVD optical disks as optical recording media having different specification, with a single optical head device. In particular, in a case of recording and reproducing an information in e.g. a CD-R employing a medium having high light-absorptivity in a 650 nm wavelength band for an optical recording medium layer, the optical head device needs to be provided with a laser diode of 790 nm wavelength band for CD in addition to a laser diode of 650 nm wavelength band for DVD.
FIG. 5 shows an example of the construction of a conventional CD/DVD compatible optical head device in which a light source unit 46 for CD in which a laser diode 35A of 790 nm wavelength band, photodetector 35B and a diffraction grating 35C are integrally formed, and a light source unit 45 for DVD in which a laser diode 34A of 650 nm wavelength band, a photodetector 34B and a diffraction element 34C are integrally formed, are separately disposed.
Light beams emitted from the laser diodes 34A and 35A are transmitted through the diffraction gratings 34C and 35C and combined into the same optical axis by a wavelength-combining prism 33, converted into parallel beams by a collimator lens 32 and converged on an information-recording plane of an optical disk 30 by an objective lens 31. The emitted light beams reflected by the information-recording plane become signal lights and converted again to parallel beams by the objective lens 31, converged by a collimator lens 30, and parts of the reflected signal lights are converged on planes of detectors 34B and 35B respectively by the diffraction elements 34C and 35C to be electrical signals.
In FIG. 5, diffraction gratings for generating tracking beams are formed on a light-source side of the diffraction elements 34C and 35C, and holographic beam splitters for converging the signal lights on photo-receiving planes of the detectors 34B and 35B are formed on the optical-disk side of the diffraction gratings 34C and 35C.
Further, such a construction is proposed, which employs a monolithic double-wavelength laser diode in which a laser diode of 790 nm wavelength band and a laser diode of 650 nm wavelength band are formed in one chip, or a double-wavelength laser diode in which laser chips for the two wavelength bands are disposed so that the distance between their emission points is about from 100 to 300 μm, whereby the number of parts can be reduced and down-sizing and low cost can be achieved as compared with conventional optical head devices in which each of the laser chips are disposed in the respective units.
This construction uses an element in which a monolithic double-wavelength laser diode in which a laser diode of 790 nm wavelength band and a laser diode of 650 nm wavelength band are formed in one chip, or a double-wavelength laser diode in which laser chips of these wavelength bands are disposed so that the distance between their emission points is about from 100 to 300 μm, and a photo-receiving element are integrally formed. In this case, the diffraction grating to be used for conventional 3-beam method or differential push-pull method and a holographic beam splitter for converging light to the photodetector, generate diffraction light of each of incident light beams of 790 nm wavelength band for CD and of 650 nm wavelength band for DVD, and the excess diffraction light becomes stray light and interfuses into the photodetector. Therefore, these arises a problem that recording and reproducing of signal can not be stably carried out.
Further, when only a diffraction grating is provided to be used for 3-beam method for CD, a light beam for DVD is diffracted by the diffraction grating for CD. Accordingly, the light intensity of light beam reaching to the optical disk is reduced, which prevents reproducing or recording of DVD type optical disk.
Further, when the diffraction grating to be used for the 3-beam method and a phase plate provided for reducing return light to minimize the variation of laser power, are respectively disposed, wavefront aberrations of the respective parts are summed up, and accordingly, there occurs a problem that the total wavefront aberration is increased.
Under the above circumstances, it is an object of the present invention to provide a double-wavelength light source unit which enables to stably detect a signal at the time of recording or reproducing an information from a CD type optical disk and a DVD type optical disk, in an optical head device employing a double-wavelength laser as a light source, and to provide an optical head device employing it.